Droplet-like Na/Vacancy Ordering Enables Ultrahigh-Na-Content P2-Type Oxide Cathodes

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Layered P2-type transition-metal oxides are key cathode materials for sodium-ion batteries (SIBs) due to high capacity and Na+ diffusion.
• However, Na deficiency in these materials leads to high-voltage phase transitions, limiting active Na+ ions and hindering practical SIB applications.

Purpose of the Study:

• To engineer a P2-type cathode with ultrahigh Na content to overcome limitations in SIBs.
• To investigate the effect of Na/vacancy ordering on the P2 framework stability and electrochemical performance.

Main Methods:

• Synthesis of P2-Na0.91Ni0.18Cu0.08Mn0.74O2 (H-Ni0.18) with a unique "droplet-like" Na/vacancy ordering.
• Systematic characterization using synchrotron X-ray techniques, neutron diffraction, and theoretical computations.
• Electrochemical testing in half-cells and full cells (H-Ni0.18//hard carbon).

Main Results:

• Achieved an ultrahigh Na content of 0.91 in the P2-cathode material (H-Ni0.18).
• Demonstrated stabilized P2 framework via Na-layer superstructure ordering, minimizing Na+ repulsion and system energy.
• Exhibited pure solid-solution reaction behavior (2.0-4.3 V) and excellent cycling stability in half-cells.
• The H-Ni0.18 cathode acted as a self-sacrificial Na reservoir in full cells, achieving 82.8% capacity retention after 150 cycles.

Conclusions:

• The "droplet-like" Na/vacancy ordering is crucial for stabilizing ultrahigh Na content in P2 cathodes.
• This strategy provides a new design paradigm for high-performance SIB cathode materials.
• Ultrahigh Na content cathodes can serve as intrinsic Na reservoirs, improving full-cell performance.